Near Infrared Photometry of Galactic Globular Clusters M 56 and M 15 . Extending the Red Giant Branch vs . Metallicity Calibration

Infrared JKs-band photometry of the Galactic globular clusters M15 and, for the first time, M56 is presented. We estimated the reddening (E(B − V) = 0.18 ± 0.08 mag) and distance modulus ((m − M)V = 15.43 ± 0.30 mag) towards the poorly studied globular cluster M56. We combined our data with observations of other clusters from the literature (12 in total) to extend the [Fe/H] vs. Red Giant Branch (RGB) slope relation towards metal-poor clusters. Our best fit yields to [Fe/H] = −3.40(±0.22) − 27.74(±2.35) × (RGB Slope), with an r.m.s. = 0.20. The broader metallicity baseline greatly reduced the uncertainties compared to other existing calibrations. We confirmed a previously obtained calibration of the relation between the RGB color (J − Ks)0(RGB) at MKs = −5.5 vs. Based on data taken at the Steward Observatory 2.3m Bok Telescope equipped with the 256x256 near IR camera.